1. Field of the Invention
The invention relates to a sensor for measuring hydrocarbon content in a gas stream in a scavenging or purging line.
2. Prior Art
A central objective in the development of modem motor vehicles is to reduce the exhaust gases which are output by the motor vehicle. In the last few years, efforts have mainly concentrated on the cleaning of the combustion gases. However, there are also a series of further sources for undesired exhaust gases in motor vehicles. These sources include the fuel tank of the motor vehicle. Fuels such as, for example, premium gasoline which are stored in the fuel tank contain a series of highly volatile hydrocarbons. These include, for example, methane, butane, and propane. These low-viscosity hydrocarbons can become released from the fuel, when there are high external temperatures in the summer or as a result of the fuel being shaken during travel, and can leave the fuel tank as a gaseous component. To counteract this, modem fuel tanks are sealed off from the outside in a gas-tight fashion. The volatile hydrocarbons must then be buffered in a hydrocarbon accumulator. Such hydrocarbon accumulators are small tanks, which are arranged above or on the fuel tank and contain, for example, an activated carbon storage element. The vaporized hydrocarbons are taken up by the activated carbon, stored and discharged again when necessary. After the activated carbon accumulator has taken up a certain quantity of the hydrocarbons, the hydrocarbon accumulator is emptied via a scavenging line. For this purpose, external air is blown into the hydrocarbon accumulator, which air takes up the hydrocarbons and carries them from the hydrocarbon accumulator to the internal combustion engine via the scavenging line. The hydrocarbons can then be fed into the intake air of the internal combustion engine and therefore contribute to the combustion. Since a certain quantity of energy is already fed to the internal combustion engine as a result of the hydrocarbons in the intake air, the injection system should inject correspondingly less fuel. According to the prior art, the valve in the scavenging line is opened according to a model stored in the engine controller, with the fuel/air mixture in the internal combustion engine being correspondingly enriched, and corresponding adjustment of the injected fuel is aimed at by means of a λ probe in the exhaust train. This control by the λ probe occurs relatively slowly, with the result that when the valve in the scavenging line opens according to the model stored in the engine controller, an air/fuel mixture which is significantly too rich is usually burnt in the engine. This leads to increased consumption by the internal combustion engine and to very poor exhaust gas values. Particularly in hybrid vehicles with very low exhaust gas emission and an internal combustion engine, which is often stationary, the problem occurs of the controlled scavenging of the hydrocarbon accumulator.